Browsing by Author "Shi, Xu"

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Cobalt Oxide (CoOx) as an Efficient Hole-Extracting Layer for High Performance Inverted Planar Perovskite Solar Cells
(2016-12-14)
Shalan, Ahmed Esmail
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Oshikiri, Tomoya
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Narra, Sudhakar
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Elshanawany, Mahmoud M.
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Ueno, Kosei
CoOx is a promising hole-extracting layer (HEL) for inverted planar perovskite solar cells with device configuration ITO/CoOx/CH3NH3PbI3/PCBM/Ag. The devices fabricated according to a simple solution procedure showed the best photovoltaic performance attaining power conversion efficiency (PCE) of 14.5% under AM 1.5 G 1 sun irradiation, which is significantly superior to those of materials fabricated with a traditional HEL such as PEDOT:PSS (12.2%), NiOx (10.2%), and CuOx (9.4%) under the same experimental conditions. We characterized the chemical compositions with XPS, crystal structures with XRD, and film morphology with SEM/AFM techniques. Photoluminescence (PL) spectra and the corresponding PL decays for perovskite deposited on varied HEL films were recorded to obtain the hole-extracting characteristics, for which the hole extracting times show the order CoOx (2.8 ns) < PEDOT:PSS (17.5 ns) < NiOx (22.8 ns) < CuOx (208.5 ns), consistent with the trend of their photovoltaic performances. The reproducibility and enduring stability of those devices were examined to show the outstanding long-term stability of the devices made of metal oxide HEL, for which the CoOx device retained PCE approximate to 12% for over 1000 h.
Correlation between Near-Field Enhancement and Dephasing Time in Plasmonic Dimers
(2020-04-24)
Li, Yaolong
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Sun, Quan
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Zu, Shuai
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Shi, Xu
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Liu, Yunquan
Near-field enhancement and dephasing time play critical roles in several applications of localized surface plasmon resonance. Here, using an example gold dimer system, we reveal the correlation between the near-field enhancement and dephasing time via time-resolved photoemission electron microscopy. Compared with isolated particles, dimers with small gap sizes show stronger near-field enhancement and shorter dephasing times. These results are well reproduced by numerical simulations and further explained by a coupled dipole approximation model. The roles of near- and far-field coupling and plasmon localization in balancing near-field enhancement and dephasing time are also unveiled.
Efficient Hot-Electron Transfer under Modal Strong Coupling Conditions with Sacrificial Electron Donors
(2019-08-01)
Cao, Yanfeng
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Oshikiri, Tomoya
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Shi, Xu
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Ueno, Kosei
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Li, Jie
The Front Cover of this special issue illustrates an investigation into hot-electron transfer efficiency under the modal strong coupling conditions. Photocurrent generation on the plasmonic photoanode was monitored with a sacrificial electron donor that consumes hot-holes efficiently. The average internal quantum efficiency at wavelengths from 500 to 800 nm reached 3%. It is emphasized, furthermore, that the Fermi level shift was observed during the hot-electron transfer involving water oxidation by the in-situ spectroelectrochemical measurements at the first time, and successfully resolve the Fermi level shift by the addition of the sacrificial electron donor. More information can be found in the Full Paper by Hiroaki Misawa et al.
Enhanced water splitting under modal strong coupling conditions
(2018-10-01)
Shi, Xu
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Ueno, Kosei
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Oshikiri, Tomoya
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Sun, Quan
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Sasaki, Keiji
Strong coupling between plasmons and optical modes, such as waveguide or resonator modes, gives rise to a splitting in the plasmon absorption band. As a result, two new hybrid modes are formed that exhibit near-field enhancement effects. These hybrid modes have been exploited to improve light absorption in a number of systems. Here we show that this modal strong coupling between a Fabry-Perot nanocavity mode and a localized surface plasmon resonance (LSPR) facilitates water splitting reactions. We use a gold nanoparticle (Au-NP)/TiO2/Au-film structure as a photoanode. This structure exhibits modal strong coupling between the Fabry-Perot nanocavity modes of the TiO2 thin film/Au film and LSPR of the Au NPs. Electronic excitation of the Au NPs is promoted by the optical hybrid modes across a broad range of wavelengths, followed by a hot electron transfer to TiO2. A key feature of our structure is that the Au NPs are partially inlaid in the TiO2 layer, which results in an enhancement of the coupling strength and water-oxidation efficiency. We observe an 11-fold increase in the incident photon-to-current conversion efficiency with respect to a photoanode structure with no Au film. Also, the internal quantum efficiency is enhanced 1.5 times under a strong coupling over that under uncoupled conditions.
Enhancement of Selective Fixation of Dinitrogen to Ammonia Under Modal Strong Coupling Conditions
(1970-01-01)
Oshikiri, Tomoya
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Shi, Xu
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Misawa, Hiroaki
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交大名義發表
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National Chiao Tung University
The production of ammonia from atmospheric nitrogen and water using sunlight is an important way to obtain ammonia as an energy carrier and is a new paradigm for achieving a low-carbon and sustainable-energy society. In this study, we attempted to enhance the ammonia photosynthesis efficiency both by increasing the electron supply using a novel photoanode composed of Au film, TiO2, and Au-NPs (ATA) with hybrid energy states due to modal strong coupling and by increasing the surface area of the Zr cathode. We investigated the key properties of this hybrid mode-assisted NH3 photosynthesis using two-electrode systems combining photoanodes and cathodes by performing photoelectrochemical measurements and product analysis. We found that the ATA structure has high absorptivity and enhances the quantum efficiency of ammonia photosynthesis and the surface area available to adsorb N-2 on the Zr cathode is a critical factor for enhancing the reaction rate.
Enhancing Surface Sensitivity of Nanostructure-Based Aluminum Sensors Using Capped Dielectric Layers
(2017-10-01)
Lee, Kuang-Li
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Tsai, Po-Cheng
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You, Meng-Lin
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Pan, Ming-Yang
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Shi, Xu
The studies of nanostructure-based aluminum sensors have attracted huge attention because aluminum is a more cost-effective plasmonic material. However, the intrinsic properties of the aluminum metal, having a large imaginary part of the dielectric function and a longer electromagnetic field decay length and problems of poor long-term chemical stability, limit the surface-sensing capability and applicability of nanostructures. We propose the combination of capped aluminum nanoslits and a thin-capped dielectric layer to overcome these limitations. We show that the dielectric layer can positively enhance the wavelength sensitivities of the Wood's anomaly-dominant resonance and asymmetric Fano resonance in capped aluminum nanoslits. The maximum improvement can be reached by a factor of 3.5. Besides, there is an optimal layer thickness for the surface sensitivity because of the trade-off relationship between the refractive index sensitivity and decay length. We attribute the enhanced surface sensitivity to a reduced evanescent length, which is confirmed by the finite difference time-domain calculations. The protein-protein interaction experiments verify the high-surface sensitivity of the structures, and a limit of quantification (LOQ) of 1 pg/mL antibovine serum albumin is achieved. Such low-cost, highly sensitive aluminum-based nanostructures can benefit various sensing applications.
Exploring the Near-Field of Strongly Coupled Waveguide-Plasmon Modes by Plasmon-Induced Photocurrent Generation Using a Gold Nanograting-Loaded Titanium Dioxide Photoelectrode
(2017-10-05)
Guo, Jingchun
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Ueno, Kosei
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Yang, Jinghuan
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Shi, Xu
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Li, Jie
Near-field spectrum measurement techniques, including near-field scanning optical microscopy, electron energy loss spectroscopy, and multiphoton photoemission electron microscopy, are powerful means to investigate near-field interactions directly on closely spaced metallic nanoparticles or a metallic nanostructure coupled with optical modes, such as whispering gallery mode and waveguide mode, which are called coupled plasmonic systems. In the present study, we have successfully measured the near-field spectra of coupled plasmonic systems using a simple photoelectrochemical measurement based on plasmon-induced water oxidation. Coupling was explored between the localized surface plasmon resonance (LSPR.) mode and waveguide mode of periodic gold nanogratings patterned on a thin titanium dioxide waveguide film. It is known that the far-field reflection spectrum of this waveguide-LSPR coupling system shows a complicated shape with multiple peaks, and the coupling has been explored by numerical electromagnetic simulations so far. However, in this study, it was clearly elucidated that an internal quantum efficiency (IQE) spectrum observed in the plasmon-induced photocurrent generation has successfully reproduced the near-field spectrum predicted by electromagnetic simulations under the coupling conditions. The photocurrent generation based on the plasmon-induced charge separation is due to the near-field effect, and it can be considered that the IQE spectrum corresponds to the near-field spectrum. This study paves a new way to indirectly measure the near-field spectrum of plasmonic systems.
Highly Sensitive Aluminum-Based Biosensors using Tailorable Fano Resonances in Capped Nanostructures
(2017-03-08)
Lee, Kuang-Li
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Hsu, Hsuan-Yeh
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You, Meng-Lin
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Chang, Chia-Chun
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Pan, Ming-Yang
Metallic nanostructure-based surface plasmon sensors are capable of real-time, label-free, and multiplexed detections for chemical and biomedical applications. Recently, the studies of aluminum-based biosensors have attracted a large attention because aluminum is a more cost-effective metal and relatively stable. However, the intrinsic properties of aluminum, having a large imaginary part of the dielectric function and a longer evanescent length, limit its sensing capability. Here we show that capped aluminum nanoslits fabricated on plastic films using hot embossing lithography can provide tailorable Fano resonances. Changing height of nanostructures and deposited metal film thickness modulated the transmission spectrum, which varied from Wood's anomaly-dominant resonance, asymmetric Fano profile to surface plasmon-dominant resonance. For biolayer detections, the maximum surface sensitivity occurred at the dip of asymmetric Fano profile. The optimal Fano factor was close to -1.3. The wavelength and intensity sensitivities for surface thickness were up to 2.58 nm/nm and 90%/nm, respectively. The limit of detection (LOD) of thickness reached 0.018 nm. We attributed the enhanced surface sensitivity for capped aluminum nanoslits to a reduced evanescent length and sharp slope of the asymmetric Fano profile. The protein-protein interaction experiments verified the high sensitivity of capped nanostructures. The LOD was down to 236 fg/mL.
Injection compression molding of transmission-type Fano resonance biochips for multiplex sensing applications
(2019-09-01)
Lee, Kuang-Li
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You, Meng-Lin
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Shi, Xu
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Li, Yi-Ru
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Ueno, Kosei
Nanostructure-based surface plasmon resonance biosensors have attracted considerable attention since the phenomenon of extraordinary light transmission in metallic nanohole arrays was discovered. However, the mass production of uniform metallic nanostructures with a low-cost, rapid, and high-throughput fabrication process remains a key issue for various multiplex sensing applications. We successfully utilized injection compression molding to mass fabricate transmission-type Fano resonance biochips with a feature size of 60 nm for multiplex sensing applications. Two types of metallic nano structures, aluminum nanoslits and capped aluminum nano slits with 24 sensing arrays, were made on polycarbonate substrates. The bulk sensitivity and uniformity of the nano structure arrays were tested. A Fano resonance with a full-width at half-maximum bandwidth of 8 nm was observed in the visible light region for 470-nm-period capped aluminum nanoslit arrays. The refractive index sensitivity was 460 nm/RIU, and a figure of merit of 58 was achieved. Moreover, aluminum nanoslits had a dip resonance with a bandwidth of 25 nm and a refractive index sensitivity of 463 nm/RIU. The coefficients of variation of the refractive index sensitivities for 24 arrays on a biochip and 10 biochips from different fabrication batches were both below 3%, which indicates that uniform nanostructures can be fabricated by injection compression molding and that the reproducibility is controllable. In addition, the multiplex sensing capability of the aluminum nanostructure arrays was demonstrated by simultaneously monitoring 24 water/glycerin solutions with a hyperspectral imaging system. Protein-protein interactions between bovine serum albumin and anti-bovine serum albumin demonstrated proof of concept of the biological detection capability of the chips. To benefit various multiplex sensing applications such as clinical disease diagnosis, drug screening, and protein biomarker discovery, 96-array aluminum nanoslit biochips the same size as a standard 96-well plate were successfully fabricated. Such a nano structure-based plasmonic biochip produced by a low-cost, rapid, and high-throughput fabrication method can benefit commercial applications. (C) 2019 Elsevier Ltd. All rights reserved.
Interfacial Structure-Modulated Plasmon-Induced Water Oxidation on Strontium Titanate
(2020-06-22)
Shi, Xu
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Li, Xiaowei
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Toda, Takahiro
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Oshikiri, Tomoya
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Ueno, Kosei
Plasmon-induced carrier transfer at metallic nanoparticle/semiconductor heterojunctions has received great attention because of its tremendous potential in applications, such as photocatalysis and photoelectric and energy conversion. The interfacial structure of the heterojunction is known to play an important role in charge transfer as well as the subsequent chemical reactions. Here, we studied the Au nanoparticle (Au-NP)-loaded (100)-, (110)-, and (111)-oriented single-crystalline strontium titanate (STO) as a model to investigate the effects of interfacial structure on the plasmon-induced charge separation between the metallic nanoparticles and semiconductors. Via photoelectrochemical characterizations, we found that the efficiency of the plasmon-induced water oxidation reaction on STO(100) is more than 1.4 times higher than that on the other two orientation facets. This enhancement was demonstrated to stem from the high oxidation ability of plasmon-induced holes captured in the surface states. Furthermore, the molecular processes of water oxidation were investigated by monitoring the surface oxidation status of Au-NP/STO as intermediates of plasmon-induced water oxidation using in situ electrochemical surface-enhanced Raman spectroscopy. The onset potential of Au-O vibrations on Au-NP/STO(100) was determined to be 0.4 V more negative than that of Au-NP/STO(110), further confirming the higher oxidation ability of the plasmon-induced holes. Our observation provides an opportunity to efficiently modulate plasmon-excited hot-carrier reaction processes for photochemical applications through interfacial engineering.
Interplay of hot electrons from localized and propagating plasmons
(2017-10-03)
Chung V. Hoang
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Hayashi, Koki
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Ito, Yasuo
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Gorai, Naoki
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Allison, Giles
Plasmon-induced hot-electron generation has recently received considerable interest and has been studied to develop novel applications in optoelectronics, photovoltaics and green chemistry. Such hot electrons are typically generated from either localized plasmons in metal nanoparticles or propagating plasmons in patterned metal nanostructures. Here we simultaneously generate these heterogeneous plasmon-induced hot electrons and exploit their cooperative interplay in a single metal-semiconductor device to demonstrate, as an example, wavelength-controlled polarity-switchable photoconductivity. Specifically, the dual-plasmon device produces a net photocurrent whose polarity is determined by the balance in population and directionality between the hot electrons from localized and propagating plasmons. The current responsivity and polarity-switching wavelength of the device can be varied over the entire visible spectrum by tailoring the hot-electron interplay in various ways. This phenomenon may provide flexibility to manipulate the electrical output from light-matter interaction and offer opportunities for biosensors, long-distance communications, and photoconversion applications.
Manipulation of the dephasing time by strong coupling between localized and propagating surface plasmon modes
(2018-11-19)
Yang, Jinghuan
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Sun, Quan
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Ueno, Kosei
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Shi, Xu
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Oshikiri, Tomoya
Strong coupling between two resonance modes leads to the formation of new hybrid modes exhibiting disparate characteristics owing to the reversible exchange of information between different uncoupled modes. Here, we realize the strong coupling between the localized surface plasmon resonance and surface plasmon polariton Bloch wave using multilayer nanostructures. An anticrossing behavior with a splitting energy of 144 meV can be observed from the far-field spectra. More importantly, we investigate the near-field properties in both the frequency and time domains using photoemission electron microscopy. In the frequency domain, the near-field spectra visually demonstrate normal-mode splitting and display the extent of coupling. Importantly, the variation of the dephasing time of the hybrid modes against the detuning is observed directly in the time domain. These findings signify the evolution of the dissipation and the exchange of information in plasmonic strong coupling systems and pave the way to manipulate the dephasing time of plasmon modes, which can benefit many applications of plasmonics.
Optimization of a compact layer of TiO2 via atomic-layer deposition for high-performance perovskite solar cells
(2017-09-01)
Shalan, Ahmed Esmail
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Narra, Sudhakar
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Oshikiri, Tomoya
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Ueno, Kosei
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Shi, Xu
We report the effect of thickness of a film consisting of a compact layer of TiO2 produced via atomic-layer deposition (ALD) for mesoporous perovskite solar cells (PSCs) with a n-i-p configuration. Uniform and pinhole-free TiO2 films of thickness from 10 to 400 nm were deposited on fluorine-doped tin-oxide substrates using ALD. The device performance of the PSC showed a trend systematic with the thickness of the ALD-TiO2 compact layer and attained the best efficiency, 15.0%, of power conversion at thickness 200 nm. Photoluminescence (PL) spectra and the corresponding PL decays for perovskite (PSK) deposited on varied ALD-TiO2 films were recorded; the effective PL quenching is due to electron transfer from PSK into the ALD-TiO2 compact layer. The most efficient interfacial electron transfer occurred at film thickness 200 nm, for which the ALD-TiO2 film has the greatest surface roughness and conductivity. We found a systematic correlation between the device performance in relation to the conductivity and the rate of interfacial electron transfer as a function of thickness of the ALD-TiO2 film; the best performance occurred at thickness 200 nm. The devices showed great stability and reproducibility, providing an alternative for high-performance PSCs with a well-controlled TiO2 compact layer.
Plasmon-induced photoelectrochemical biosensor for in situ real-time measurement of biotin-streptavidin binding kinetics under visible light irradiation
(2017-03-08)
Guo, Jingchun
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Oshikiri, Tomoya
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Ueno, Kosei
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Shi, Xu
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Misawa, Hiroaki
We developed a localized surface plasmon-induced visible light-responsive photoelectrochemical (PEC) biosensor using a titanium dioxide (TiO2) photoelectrode loaded with gold nanoislands (AuNIs) for in situ real-time measurement of biotin-streptavidin association. As a proof of concept, self-assembled thiol-terminated biotin molecules bound on a AuNls/TiO2 photoelectrode were successfully utilized to explore the photocurrent response to streptavidin-modified gold nanoparticle (STA-AuNP) solutions. This plasmon-induced PEC biosensor is simple and easy to miniaturize. Additionally, the PEC biosensor achieves highly sensitive measurements under only visible light irradiation and prevents the UV-induced damage of samples. Furthermore, a novel approach has been proposed to realize the real-time monitoring of biotin-STA binding affinities and kinetics by analyzing the PEC sensing characteristics. This PEC biosensor and novel analysis method could provide a new approach for the specific electrical detection and real-time kinetic measurements for clinical diagnostics and drug development. (C) 2016 The Authors. Published by Elsevier B.V.
Solid-State Plasmonic Solar Cells
(2018-03-28)
Ueno, Kosei
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Oshikiri, Tomoya
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Sun, Quan
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Shi, Xu
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Misawa, Hiroaki
Metallic nanoparticles such as silver and gold show localized surface plasmon resonances (LSPRs), which are associated with near-field enhancement effects in the vicinity of nanoparticles. Therefore, strong light-matter interaction is induced by the near-field enhancement effects of LSPRs. Because the resonant wavelength of LSPRs can be easily controlled by the size and shape of the metallic nanoparticles in the visible and near-infrared wavelength range, LSPRs have received considerable attention as optical antennae for light energy conversion systems such as solar cells. LSPRs decay very quickly as a result of light scattering and excitation of electron-hole pairs in the metal itself. However, in addition to the near-field enhancement effect, this light scattering and electron-hole pair excitation, which are known to cause loss of LSPRs, can be utilized as a solar cell enhancement mechanism. Here, we focus on plasmonic solid-state solar cells. The mechanisms of the light scattering by LSPRs, near-field enhancement, and plasmon-induced charge separation based on electron-hole pair excitations can be clarified. We review the related studies from the viewpoint of these mechanisms rather than material science.
Water splitting using a three-dimensional plasmonic photoanode with titanium dioxide nano-tunnels
(2017-05-21)
Takakura, Ryohei
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Oshikiri, Tomoya
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Ueno, Kosei
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Shi, Xu
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Kondo, Toshiaki
In this study, we developed a three-dimensional plasmonic photoanode using titanium dioxide nanotunnels (TNTs) loaded with gold nanoparticles (Au-NPs) for water splitting, to enhance the reaction efficiency. We also optimized the procedure of loading Au-NPs on complex three-dimensional structures. We discuss the correlation between the plasmon-induced charge separation obtained from photoelectrochemical measurement and the morphology of Au-NPs observed by transmission electron microscopy. We have successfully deposited well-dispersed Au-NPs on the walls of TNTs using HAu(OH)(4) as a precursor. The amount of Au-NPs on the TNTs was estimated to be approximately 15-fold larger than that on the thin film titanium dioxide substrate, and the particle size remained small. Photoelectrochemical water splitting was achieved by using a two-electrode system rather than a three-electrode system. Furthermore, stoichiometric water splitting was confirmed by estimating the amounts of the evolved H-2 and O-2 gases under visible light irradiation.